Most semiconductor devices are intolerant of excessive voltage; destructive breakdown or damage can occur in semiconductor devices when subjected to transient overvoltages that persist for only a few microseconds. Transient Voltage Surge Suppression (TVSS) devices, referred to interchangeably herein as surge suppressors and voltage-clamping devices, are commonly known for use in suppressing such overvoltage transients to protect voltage-surge intolerant circuitry. TVSS devices commonly include nonlinear, voltage-dependent resistive elements which display electrical behavior similar to that displayed by a pair of series-connected, back-to-back zener diodes. At normal voltages, below the TVSS clamping voltage level, TVSS devices display a high resistance with a small leakage current. When subjected to large transient voltages (above the TVSS device's clamping voltage), the TVSS device is forced by its characteristics to operate in a low resistance region which increases current flow through the device. The increased current produces an increased voltage drop across the source impedance, effectively clamping the transient voltage to a level determined acceptable (i.e., safe) for the protected circuit. The potentially destructive surge energy is thereby dissipated or passed through the voltage-clamping (TVSS) device and its operating current returns to its normal range after the surge.
Avalanche diode suppressors, metal oxide varistors (MOVs) and selenium surge suppressors may be utilized as TVSS devices with varying advantages and disadvantages, such as an MOV-based TVSS's inherently susceptibility to failure under certain conditions. More specifically, MOV components have a tendency to explode when overheated, often with sufficient explosive power to fracture plastic housings and sheet metal enclosures within which they reside. The explosion usually completely destroys everything within the TVSS housing and may possibly shoot hot black powder through any small openings in the housing. Various techniques have developed to protect MOVs from the causative factors leading to such explosive conditions.
One technique for protecting metal oxide varistors (MOVs) requires adding a current fuse in series with the MOV, which trips to an open state to protect the MOV when particular transient overvoltages are detected. Transients with I.sup.2 t ratings that are greater than the fuse rating, but just below the MOV rating will blow the fuse, electrically removing the MOV from the overvoltage condition. Under circumstances where the fuse displays an I.sup.2 t rating such that commonly occurring transients are insufficient to blow the fuse, i.e., from a few to 10,000 amperes, but of insufficient magnitude to force the MOV to its low impedance state, the MOV is subjected to overheating, possibly leading to thermal runaway. Steady state, abnormal overvoltage conditions below those at which the fuse will blow may also generate sufficiently high currents through the MOV leading to dangerous overheating.
A second common technique for protecting MOVs from overheating due to abnormal steady state or transient overvoltage conditions utilizes a thermal cutoff device (TCO) provided electrically in series with the MOV. TCOs sense the surface temperature of the MOV and trip to a high impedance state (open circuit) at a particular maximum rated temperature, cutting off voltage to the MOV. Thermal cutoff devices, however, like current fuses are not without problems when used within MOV-protected circuits. In particular, it is extremely difficult, and sometimes impossible to achieve good thermal contact between a surface of the MOV and a thermal cutoff device. Consequently, the MOV may overheat to a point of thermal runaway before the critical temperature is detected and the overvoltage is cut-off from the MOV by the TCO. Further, mismatch problems may also occur between the time constant of the thermal cutoff (i.e., time to blow) and heating/time characteristics of the MOV even when good temperature detection is possible, rendering accurate MOV protection unreliable. And in addition, both current fuses and temperature cut offs are permanently opened so that although explosions may be prevented, the transient voltage surge suppressor function is permanently lost.
It would therefore be desirable to have available an MOV overvoltage protection circuit which is effective in reliably cutting off voltage to the MOV to prevent damaging overheating in the MOV, after which the MOV is connected back into the circuit. Preferably, the MOV protection circuit would effectively protect the MOV whether the overvoltage conditions are continuous or temporary (i.e., transients). It would also be desirable to have available an MOV overvoltage protection circuit which cuts off voltage across the MOV in accordance with a length of time for which the MOV is subjected to a specific overvoltage level, and the MOV's thermal time constant. After a time period defined according thereto is run, the protection circuit would disconnect the MOV from the AC source. The time of cutoff would theoretically be just prior to the point in time at which the MOV would have failed. When the temperature of the MOV is detected to be at a safe MOV operating level, the MOV would be coupled back into the circuit thereby minimizing a time in which the MOV is electrically removed from the circuit it was meant to protect.